The invention relates to a light emitting diode device comprising a light emitting diode arranged on a substrate, and a wavelength converting element containing a Mn4+-activated fluoride compound. The invention also relates to a luminescent material as well as to a method for preparing such luminescent material.
Light emitting diode devices (abbr. as LED devices) are widely known as new semiconductor light sources with promising lighting properties for future applications. These LED devices should eventually substitute many of the current light sources, like incandescent lamps. They are especially useful in display lights, warning lights, indicator lights and decoration lights.
The color of the emitted light depends on the type of semiconductor material. LEDs produced from Group III-V alloys—such as GaN—are well-known for their ability to produce emission in the green to UV range of the electromagnetic spectrum. During the last decade, methods have been developed to convert (parts of) the radiation emitted by such ‘blue’ or ‘(near)UV’ LEDs into radiation of longer wavelength. Phosphors are widely used luminescent materials for this purpose. These phosphors are crystalline, inorganic compounds of high chemical purity and precisely controlled compositions. They comprise small amounts of specifically selected elements (‘activators’), which make them to efficient luminescent materials.
In addition to colored LEDs, the development of so-called ‘white light LEDs’ is also very important. An interesting configuration in this field is based on converting a part of the light generated by a blue/UV LED and mixing that converted part with the non-converted part of said generated light, so obtaining white or white-like light. In this area blue emitting GaInN LEDs are most popular. Ce3+-activated Yttrium Aluminum Garnet (YAG-Ce) and Eu2+-activated Ortho Silicates (BOSE, OSE) are well-known phosphors for this purpose.
A LED device as described in the opening paragraph is known as such, for example from the patent publication WO 2009/012301-A2. This document describes in great detail a number of LED devices in which Mn4+-activated fluoride compounds are applied as a luminescent material in the wavelength converting elements of these devices. Emission and excitation spectra of a number of K2[XF6]:Mn4+ (X=Nb or Ta) and K3[XF7]:Mn4+ (X=Bi, Y, La or Gd) phosphor compounds are shown. These luminescent materials appear to show a narrow band or line emission in the red spectral region (600-660 nm) of the electromagnetic spectrum. This is very attractive as LED devices comprising such luminescent materials in their wavelength converting elements are able to produce ‘warm white’ light. This is light with a comparative color temperature (CTT) below 5000K.